In the field of signal processing, it is of the utmost importance to be able to separate actual signals from noise. Electronic filters are used to modify the characteristics of an incoming signal so as to provide an output signal which is modified in some defined fashion. In the field of target detection devices such as radar and sonar, a pulse is sent out in a certain direction whereupon the pulse is reflected on, for example, a target and whereupon the reflected pulse is received by a receiving unit. The receiving unit transforms the received pulse into an electronic signal, for example a complex video signal. The video signal is then processed by the utilisation of a number of components, one of which is a filter. The pulse that is sent out is normally coded in order to enable suitable signal processing on the video signal. The coding may be in the form of for example, frequency coding or binary coding. The filter is then chosen such that the filter utilises the coding for different tasks. One such task is pulse compression which is done by adapting the filter to the coding, and is utilised in order to gain a better range resolution for long pulses.
Pulse compression is well known in prior art, as is the problem of forming of side lobes when using pulse compression. This characteristics of the filter is crucial regarding the forming of side lobes. The shorter the filter the higher the side lobes before and after the main lobe. However, the shorter the filter the shorter the range of side lobes before and after the main lobe. The shorter filter however yields a better sensitivity than a longer filter, especially when it is matched to the outgoing pulse coding filter. The previously known filters have to be adapted in regard to the trade off between the sensitivity and the presence of the side lobes. If the filter is too short, the side lobes will prevent possible detection of weak return signals (in the side lobe ranges/regions) near a strong signal, i.e. the ratio between the main lobe amplitude and the side lobe amplitude is very small. However, if the filter is long the ratio between the main lobe amplitude and the side lobe amplitude is high, but with the disadvantage of an extended side lobe range. The ratio may be discussed in terms of main lobe level (MLL) versus side lobe level (SLL), where a relative increase in the SLL to the MLL gives a decrease in the ratio and vice versa.
U.S. Pat. No. 5,502,747 teaches a digital filter with long impulse response and low latency using Fast Fourier Transformation or Modified Discrete Fourier Transformation. The filter comprises a number of components in parallel and operates by combining their outputs by addition. U.S. Pat. No. 5,502,747 does not teach anything that remedies the trade off problem discussed above, but the problem of choosing a suitable filter length still prevails.
U.S. Pat. No. 4,359,735 teaches a digital pulse compression processor for reducing the processing loss in target-echo signals in a radar or sonar system. The processor uses pulse compression on two channels with clock signals on the same and predetermined Nyqvist frequency. The second channel has its pulses interlaced in time, approximately midway between the pulses of the first clock signal. According to the document, the interlaced sampling periods give rise to reduced sampling error. The document teaches addition of I and Q signals on both channels and multiplication of the added signals after taking the square root on each added signal. U.S. Pat. No. 4,359,735 does not teach anything that remedies the trade off problem discussed above, but the problem of choosing a suitable filter length still prevails.
Hence, there still remains a need for a better signal processing arrangement and a better method when using pulse compression in order to increase the MLL to SLL ratio with an acceptable sensitivity and range resolution.